Microfluidic devices are used for a variety of different purposes, such as assaying, so that less fluid is used and automation can be increased, both of which serve to decrease costs. A consequence of the smaller fluid volumes and the smaller channels and reservoirs within such microfluidic devices is the resulting domination of capillary, or surface tension, forces over the fluids deposited within the devices. As such, fluid delivered into a reservoir of a microfluidic device may undesirably wick into a connecting channel, resulting in lost reagent volume, contamination, and other problems. Existing approaches to resolve this issue typically employ passive (or capillary) valves, or active valves. However, the former type of valve at least occasionally fails, while the latter type of valve can be prohibitively expensive to incorporate within a microfluidic device.